There are things out there, in the weirder reaches of space-time where reality is an optional extra. Horrible things, usually with tentacles. Al-Hazred glimpsed them, John Dee summoned them, HP Lovecraft wrote about them, and Alan Turing mapped the paths from our universe to theirs. The right calculation can call up entities from other, older universes, or invoke their powers.
Thats where the Laundry comes in its a branch of the British secret service, tasked to prevent hideous alien gods from wiping out all life on Earth (and more particularly, the UK). You work for the Laundry. You may get to save the world.
Just make sure you get a receipt.